The present invention relates to the design of a seal gland, especially but not exclusively to the design of a gland for a rotary seal in a remote environment.
Background: Bearing Seals
In applications in which relative motion is necessary, one of several types of bearings are used, such as ball bearings, roller bearings, or more simply journal bearings. A seal, such as an elastomeric seal, is typically used between the bearings and the outside environment to keep lubricant around the bearings and to keep contamination out. In a rotary seal, where one surface rotates around another, some special considerations are important in the design of both the seal itself and the gland into which it is seated. For instance, the elastomeric seal should be under compressive (never tensile) stress, and while there should be enough pressure between the seal and the rotating surface to prevent leakage, the pressure should be minimized to reduce friction and wear. Additionally, there should be enough room in the gland to allow for expansion under changing conditions but not excessive room which could allow the seal to twist or buckle.
Additional information regarding seals can be found in Practical Seal Design, Leonard J. Martini (1984) and in Seals and Sealing Handbook, Fourth Edition, M. Brown (1995), both of which are hereby incorporated by reference.
Background: Drill Bits
One of the important types of rotary drill bits in the petroleum business is the roller cone bit, seen in FIG. 6. In such bits, a rotating cones 42 with teeth 44 on its outer surface is mounted on an arm 46 of the drill bit body. FIG. 7 shows a drill rig, including the drill bit and a drill string, which includes sections of pipe which transfer rotational force to the bit and which carry drilling fluid to the bottom of the hole, where it washes out debris. As the drill bit rotates, the applied weight-on-bit ("WOB") forces the downward pointing teeth of the rotating cones into the formation being drilled. Thus the points of the teeth apply a compressive stress which exceeds the yield stress of the formation, and this induces fracturing. The resulting fragments are flushed away from the cutting face by a high flow of drilling fluid, referred to as "mud".
Although improvements have been made in roller-cone-type bits over the years, improvements have continued to be needed in the seals which protect the bearings. The constraints on the seals used in these applications are different from those of other low-speed sealing applications in several respects. First, everything in a bit, which operates deep in the earth, must be extremely robust to withstand the pressure and eccentric motion to which the bits are subjected. Additionally, the seals are themselves exposed to abrasive materials from two sources: not only does the drilling fluid near the cutting face include a heavy load of abrasive material (which is moving very turbulently at very high velocities), but the bearings themselves, as they wear, will tend to produce metal particles, and these metal particles themselves may be abrasive to a soft seal. Thus, both sides of the seal should ideally be protected from these abrasive effects. Additionally, the bit is operating in a remote environment from which it may take hours to retrieve for replacement, so it is highly desirable to have the bit operate for as long as possible.
For sealing on a rock bit, an O-ring, or a derivative of O-ring, are typically used. One problem with this seal is that, as the bit is operated, the seal will inevitably wear, so that less compressive force is applied against the moving surface, running the risk that a leak will develop across the seal.
One previous gland design is seen in U.S. Pat. No. 4,372,624 to Neilson and is reproduced in FIG. 8. In this patent it is seen that an O-ring of circular cross-section is confined within a pair of symmetrical and complementary V-shaped surfaces having rounded vertices, one V-shaped surface being formed on a bearing journal within the body of a rock bit, the other V-shaped surface on the cutter cone mounted on the journal. This design allows the O-ring to move axially in response to differential pressure across the seal, with movement in either direction causing an increase in the squeeze on the seal. Reserve squeeze is provided if and only if needed.
Another type of gland design is seen in U.S. Pat. No. 5,129,471 to Maurstad et al., and is reproduced in FIG. 9. In this gland, two walls are in the moving cone, while the other two are part of the stationary journal. The seal rides between surfaces 45 and 46 having flat cross-sections, with a shroud or protrusion 48 to bias the O-ring away from wall 49 and prevent wear.
Improved Seal Gland Design for Low-Speed Rotary Application
The present inventors have noted that for a given amount of squeeze on the seal, the contact pressure across the sealing surface is minimum when the seal is compressed between two surfaces whose cross-section is flat, as compared to a V-surface or any other surface with a radius. This means that while the gland of FIG. 8 provides advantages in the latter portions of the seal lifetime, it does not provide optimum sealing in the early part of the seal's use.
The present application discloses a seal gland whose shape provides two modes of operation: in the first part of its lifetime, the seal is seated on a base portion of the gland, which is concentric with the journal sealing surface, so that it functions analogously to a normal elastomeric seal, providing adequate compression as installed; but as the sealing element wears and contact pressure is lessened (resulting in more axial movements with either cone vibration or a small pressure differential), the axial movement of the seal causes it to be further compressed by a chamfer within the gland. Unlike the previous attempts to solve this problem, this solution seeks to optimize the function of the seal while minimizing the wear in both portions of its lifetime.
In some embodiments, the present application discloses a seal having a cross-section which is roughly circular, but with a recess in the side on which movement occurs, so that lubricant can be provided therein.
In some embodiments, this application further discloses preventive measures to protect the seal and lubricant from contamination. A groove trap can be placed circumferentially on the cone or arm journal bearing or on the seal boss surfaces. This trap collects and stores bearing wear materials that are generated during cone rotation, keeping them out of the sealing surfaces.
In some embodiments, this application further discloses a filter on both sides of the sealing surface to prevent both cutting and bearing materials from contaminating the seal surface. This filter takes the form of another ring of a softer elastomeric material, which will inherently wear slower than the seal itself.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:
lifetime of the seal is improved; and PA0 the correct amount of pressure is maintained on the seal during its lifetime.